The composition of exhaust produced by the combustion of hydrocarbon fuels is a complex mixture of oxide gases (NOX, SOX, CO2, CO, H2O), unburned hydrocarbons, and oxygen. Measurement of the concentration of these individual exhaust gas constituents in real time can result in improved combustion efficiency and lower emissions of polluting gases. Various devices have been proposed to operate as exhaust gas sensors that have the capability of measuring the concentration of a gas constituent in an exhaust stream. Many of these devices require the flow inside the sensor to be regulated and/or maintained within a certain allowable range. In order to help ensure accuracy of the devices, the sampling conditions of the exhaust gas should be precisely controlled.
The sampling conditions can include a sampling rate and/or an amount of gas flow provided to a sensing element. For example, a low exhaust pressure external to the sensing element may hinder the necessary amount of gas from contacting the sensing element. Also, the flow of exhaust gas monitored by such sensors typically has pulsations in its flow rate caused at least in part by engine cylinder firings. These pulsations have been shown to result in measurement error. Furthermore, a catalyst structure may also be employed to improve sampling conditions by converting one or more constituents into another constituent prior to measuring the constituents.
One method of regulating the flow of exhaust through a gas sensor is described in U.S. Pat. No. 6,015,533 (the '533 patent) issued to Young et al. on Jan. 18, 2000. Specifically, the '533 patent discloses a cylindrical sensor housing having an inner shroud and an outer shroud that directs the flow of exhaust into and out of the sensor housing. Gas flows into the sensor housing via orifices located within cylindrical walls, at a tip end of the outer shroud. The gas then travels through gas channels running longitudinally, with respect to the cylinder walls, between the inner shroud and the outer shroud. At a proximal end of the inner shroud, the gas passes through a diffuser, reverses direction, and passes through a chamber defined by the inner shroud and a sensing device. The gas then passes over a tip end of the sensing device and out of the sensor housing via a port located at a distal end of the outer shroud, substantially perpendicular to the orifices.
Although the sensor housing of the '533 patent may suitably regulate gas flow through a sensor, the sensor design may have some limitations. Specifically, the complexity of the sensor may increase the costs associated with manufacturing and distribution of the sensor. The presence of the diffuser may render the sensor susceptible to clogging due to soot present in the exhaust. And, the diffuser may be difficult to access and therefore costly to service and/or replace. Furthermore, the '533 patent includes, nor allows for, a catalyst structure to filter and/or condition the exhaust prior to the exhaust contacting the sensing device. This may hinder the reliability of the readings acquired by the sensing device.